For LTE (Long Term Evolution) TDD (Time Division Duplex), the inter-cell interferences of eNB (evolved NodeB) to eNB or HeNB (Home eNB) to HeNB and UE (User Equipment) to UE are related to the cell synchronization. In order to overcome the above interferences, strict synchronization is required. The synchronization requirement for a HeNB is defined as the difference in radio frame start timing, measured at the transmit antenna connectors, between the HeNB and any other HeNB or eNB which has overlapping coverage. The synchronization requirement shall be set to 3 μs in all cases, except when the HeNB gets its synchronization when performing network listening of cells with propagation distance greater than 500 m. This requirement shall apply independent of the synchronization technique used (i.e. GPS (Global Positioning System), IEEE 1588 v2, Network Listening), as defined in document 3GPP TR 36.922.
Small cell solution is becoming more and more important in the LTE network both for coverage, capacity and traffic offloading. For TD-LTE (Time Division-LTE) system, the synchronization technology of small cells is critical because of the DL/UL (Downlink/Uplink) interference issue. Considering the GPS signal strength of the indoor environment and the network support of the IEEE 1588v2, network listening has been considered as the essential method embedded in the small cells (not only femto but also pico or other small cells without GPS).
Among the above mentioned three synchronization techniques, network listening can be used in scenarios where GPS and IEEE 1588 v2 do not work. For this reason, network listening is an essential synchronization scheme for TD-LTE HeNBs in those scenarios. The technique in which a HeNB derives its timing from a synchronized eNB or HeNB (which in turn may be GNSS (Global Navigation Satellite System)-synchronized) is referred to as “synchronization using network listening”.
In the network listening mode, the HeNB may periodically track one or more signals from the donor cell. Currently, in 3GPP TR 36.922 there are two fully backward compatible schemes for tracking the Common Reference Signal (CRS) which have been proposed: one that uses MBSFN (Multicast Broadcast Single Frequency Network) subframes and one that uses the guard period between DL and UL transmission.
Actually the network listening is not only required by the HeNB but also by any small cell without GPS or IEEE 1588 v2. In a recently presented concept called ‘Flexi Zone’, the Heterogeneous Network architecture will include both the indoor and outdoor small cell clusters for traffic offload and capacity solution. In a LTE TDD network, the synchronization between the neighboring small cells would be critical.
Currently, both alternative network listening schemes proposed in 3GPP depend on the CRS tracking, either in particular MBSFN subframes or in guard period between DL/UL switching. According to the two algorithms, whenever the CRS tracking fails, the listening cell has to turn off the Tx (Transmitter) and restart the cell search process, i.e. the HeNB works as a UE to search the synchronization signals from the neighbouring cells. This would cause a restart of the cell and all the UEs inside the cell will be affected. Therefore, a high frequency of cell search process would result in a very bad user experience.
Practically considering the complex indoor wireless environment and unexpected changes of the environment between neighbouring cells, especially if the small cell is outdoor or in some hot spot, the CRS tracking may fail due to several different reasons:                1) the donor cell is off;        2) the signal transmission environment between the donor cell and the listening cell is changed permanently so that the listening cell could not find the donor cell.        3) the signal transmission environment between the donor cell and the listening cell is changed dynamically due to some temporary interference or shielding source in a short period.        
It is noted that the third scenario mentioned above may happen frequently in a real network. Based on a TD-LTE femto eNB prototype developed by the applicant of the present application, and by implementing the network listening algorithm, it has been verified in real test that the CRS tracking may be dropped for a short while dynamically.
FIG. 1 schematically shows a Donor HeNB 10 having an antenna 11 and a listening HeNB 20 having an antenna 21. When the listening HeNB and Donor HeNB are set close enough to each other, the CRS tracking worked quite stable with a synchronization accuracy as high as ±100 ns. However, when the distance between the two cells is increased up to 20 meters or more across an office room, the CRS tracking was observed to be often dropped for a short while due to the disturbing environment, e.g. caused by walking people in between or the like.
According to the current algorithm described in 3GPP, both of the two schemes will suffer from the frequent restart problem due to the CRS tracking failure in scenario 3 described above. Therefore, a robust method to solve this problem is highly recommended.